This invention relates to a system for automatically detecting in a rotary printing press incipient printing faults on a metallised packaging strip, such as aluminised film, or any other substrate having a high reflecting power.
Frequently used in the packaging industry for printing strips fed from a reel, machines of this kind comprise a plurality of constituent stations, namely, progressing logically from upstream to downstream with reference to the direction of movement of the strip, a feed station comprising a reel holder and an automatic strip connector followed by a strip accumulator as required for each connection, an introduction station comprising a strip straightener and guide, a sequence of one or more printing units provided with dryers, and finally a reel receiving station or, if required, a station which directly introduces the printed strip into a new machine enabling it to be cut either by rotary working or flat working.
The automatic printing fault detection system will be used after this latter unit. Each of the printing units can give rise to different faults which may be in the form of streaks, smudges or blotches, variations in print intensity, print holes, or poor register in the case of a fault due to a shift between the different printing colours.
Numerous display and/or detection systems for these faults do exist in the prior art and generally use a video camera or any other means of picking up the light reflected by the printed strip. However, all these systems are intended primarily for monitoring non-specular colour prints on matt supports which have no particular brightness capable of reflecting the light in a given direction. It is therefore advantageous to differentiate the printing supports and inks designated as matt, in which it is not possible for any image to be reflected, from the metallised printing inks and supports whose reflecting power is associated with the aluminium foils typically used for perishable foods.
When a beam of light illuminates a non-reflecting surface termed matt, the light which is returned by this surface is a diffuse light which is reflected in every direction. On the other hand, if this surface is that of a metallised strip, the incident beam of light will be reflected as in a mirror in a given direction, at an angle of reflection equal to the angle of incidence of the beam. The difficulties that are found when monitoring a metallised strip depend essentially on the actual nature of the strip which has a specular and non-diffusing reflecting property. The subsequent problem in respect of optics or lighting and the sensing, by a camera, of the light reflected by the strip, form the subject of this invention inter alia. To obtain a good image quality, either with good colour rendering or a faithful image of the printed pattern, and avoid any problems in respect of deformation, lack of clarity and non-uniformity of the luminous flux sensed at the surface of its field of view, the camera is advantageously placed along a photographing axis oriented perpendicularly to the plane of the metallised strip. Since the illumination of a metallised strip can be likened to illumination of a mirror, the arrangement of a camera opposite said strip will naturally have the effect of giving rise to reflection of the image of the camera by the metallised strip. Although the camera may for the major part be hidden behind an opaque mask, the camera lens cannot be masked similarly in any case whatsoever. The geometric configuration of the camera lens with respect to the plane of the metallised strip is such that, in view of the specular properties of the strip, the camera can only relentlessly film the image of its own lens. Since the latter is directly connected to the camera photographic chamber, it follows that a black disc having a contour of varying definition depending on the quality of the reflection of the metallised strip will permanently appear at the centre of the image given by the camera. In order that the rays of light reflected by the metallised strip may traverse the lens of the camera in accordance with such a configuration, said rays must originate from a light source disposed on the same axis as the camera lens by virtue of the law of reflection which states equality between the angle of reflection and the angle of incidence of a ray of light. If that is not the case, the rays of light will not traverse the camera lens, and will therefore give rise to a darkened image of the strip on which the metallised surfaces will appear as surfaces of a substantially black tint. This blanking out of the image of the metallised parts of the strip, which is quite specific to such materials or inks having a predominance of specular colours, means that it is no longer possible to exclude any printing fault over the entire monitored surface.
To obviate this problem, various known systems making up the prior art provide a solution, depending on specific needs, to the problems of shade or reflection caused by illumination of strips subject to print or sealing quality control in a specific case.
For this purpose, the patent JP 9 300 596 describes a lighting system enabling a strip of any substrate to be inspected and printing faults to be detected. The system in question comprises a fixed lighting comprising three light tubes disposed opposite the front side of the vertically moving strip, and a light tube which detects the print holes by illuminating the back of the strip. Opposite the strip, two or three lights are disposed symmetrically on either side of its perpendicular and illuminate at an angle of 55xc2x0 with respect to its surface. These two lights contribute an illumination intended to be reflected by all the diffusing surfaces. The beam of light from the third light is in turn intended to be reflected by specular surfaces. Its inclination with respect to the perpendicular to the strip is 8xc2x0. A camera situated opposite the strip standing back from the light fittings films the strip at an angle of the same value so that the reflection of the latter beam traverses the camera lens if it is reflected by a specular surface. Also, the inclined plane defined by the lamp for the front of the strip is such that it includes the axis of the camera lens. The four light tubes operate simultaneously and their lighting power is individually determined and monitored by an electronic monitor.
Another system is given by the patent EP 781 655, in which the inventor proposes apparatus and a method whereby during optical quality monitoring it is possible to reduce the shade effects which typically form on a strip formed by a transparent support moving horizontally above a surface having a diffusing opaque background. Comprising in particular transparent parts and opaque parts, said strip is illuminated on its front surface by an oblique parallel beam of light. The oblique arrangement of the beam is necessary in view of the position of a camera situated opposite the front surface of the strip on an axis perpendicular to the latter. In order to avoid static electricity problems as a result of friction of the strip on the opaque background surface, an indispensable gap separates the moving strip from the background surface. Since the lighting beam incidence to the printed strip is not perpendicular, a shade zone of triangular section proportional to the height of the gap and to the value of the angle of inclination of the incident beam inevitably forms in the gap and around all the opaque surfaces of the strip. To meet this problem, the inventor proposes that the opaque background surface should be replaced by a support consisting of a plurality of layers of a material which diffuses and reflects the light by cascade from one layer to the next. In this way there is created, in depth in each of the layers of said support, a better distribution of the light in every direction contributing to eliminate or attenuate the unwanted shade effect.
The patent JP 4 071 849 describes an optical detector designed for inspection of printed strips, including those having surfaces with a high reflecting power. Moving horizontally, the strip is illuminated obliquely on its front surface by a light tube. The light reflected by the strip is returned in the direction of a plurality of sensors disposed in line above the examined surface. In front of each of these sensors is a dissector polarising filter which allows extraction, from the beam reflected by the strip, of all the rays which have undergone specular reflection, i.e., all the rays whose angle of incidence is equal to the angle of reflection. Thus monitoring of the patterns printed on the strip is determined solely by the processing of a beam of a light entirely diffused by the surface thereof.
The patent JP 4 203 955 presents another variant for examination of a printed strip, which eliminates all the interference due to unwanted specular reflection. This method is based on converting an analogue signal proportional to the luminous intensity of a light reflected by the strip, into a binary signal coding for the examined image portion. Since the rays reflected as a result of specular reflection have a higher luminosity than those which have been diffused by the strip, it is then possible to differentiate these latter rays from the former by establishing an intensity threshold which limits the sensed analogue signals.
The patent JP 3 255 346, by monitoring an aluminised strip formed in this case by a packaging for pharmaceutical tablets, solves an optical problem of detecting faults in sealing a metallised strip on circular cavities containing pharmaceutical pills. The system in question has the feature of being able to detect faults without being affected by the wave-form corrugation on the aluminised back of each of said trays of tablets, following the normal production process used. Moving horizontally, the aluminised back faces a lamp which illuminates said surface obliquely, the strip reflects the incident rays with an irregular intensity depending on the distribution of the waves on its surface. The image of the luminous intensity of such a surface without any fault is previously stored and taken as a model by an electronic monitor. The rays reflected by the metallised strip are projected on to a horizontal translucent diffuser screen behind which the lens of a camera is situated. Obtained by transparency through the screen, the image given by the camera is compared by the electronic monitor with that which was taken as reference. Thus any fault in the sealing of the metallised strip on the cavities gives rise to a local luminous intensity different from that of the model, and allows damage to be detected in the sealing of the packaging under examination.
Due mainly to the diversification of the applications or objects to which these systems relate, the systems nevertheless give rise to a number of disadvantages which are, inter alia:
lack of specular and diffusing colour rendering and fidelity for an objective comparison,
the fact that in all the systems which do not differentiate between the illumination of the diffusing surfaces and the illumination of the specular surfaces, there will inevitably be, between these two types of surfaces, an imbalance in luminosity resulting in over-intensity of the metallised colours to the detriment of the contrast of the diffusing colours,
the use of an oblique illumination for monitoring the reflecting surfaces, and this involves a double disadvantage, namely: a greatly reduced extent of the field of view of the camera because of the fineness of the beam of light which can traverse the lens after specular reflection, and a distortion of the image given by the camera because its photographing axis is oblique to the perpendicular to the monitored surface,
the fundamental impossibilityxe2x80x94in view of the objective to which the subject matter of this invention relatesxe2x80x94of having access to a process intended to eliminate all the beams directly reflected by the examined surface and which have not undergone any diffusion by the same,
the fact that certain systems are ineffective for processing transparent or translucent strips although they contain metallised surfaces,
the use of a detection method which is poorly adapted, if at all, to the detection of printing faults on flat metallised strips of various colours without any particular corrugation.
The object of the system according to the invention is to obviate these disadvantages by providing a rapid tool of high performance in detecting all kinds of faults both during quality control in respect of the production of strips having a predominance of specular colours and those which have matt impressions. Of course this system also enables mixed strips to be processed, such strips partly comprising metallised surfaces or surfaces having a high reflecting power, and partly surfaces of diffusing colours. One of the features of this system is to improve the visual appearance of the reflecting surfaces filmed by a camera so that on the one hand the latter do not appear as being black or greatly faded zones in which any faults cannot be made visible, and on the other hand so that all the diffusing surfaces, even the dark ones, printed on an aluminised substrate, can be perfectly distinguished on a video monitor screen. It is by applying these specific features that it is possible to automate the detection process without any fear of blanking outxe2x80x94or certain darkeningxe2x80x94of the system which would immediately result in a considerable reduction of the reliability of the detection system in view of the impossibility of being able to detect numerous faults which have already been qualified as inadmissible.
While inter alia allowing monitoring of the texture of metallised surfaces thanks to the detection of faults which may range down to a minimum size less than 0.1 mm, depending on the camera characteristics, the system according to this invention has the advantage of being able to detect in real time incipient faults on all the reflecting or diffusing surfaces during printing of the strip. In this way it is possible to detect at a very early stage and automatically any printing fault which may assume unacceptable dimensional proportions at varying speed, thus avoiding certain and considerable wastage in production. Finally it should be noted that irrespective of the choice of strip used, this system also equally well enables detection of clear and contrasted faults which are readily visible to the naked eye once located, and faults which are distinctly less marked, which merge into their surroundings due to a certain blurring or camouflage imparted by colours of relatively low contrast.
To this end, the invention relates to an automatic printing fault detection system suitable inter alia for monitoring metallised strips and in accordance with the text of claim 1.
In order to define some of the terms introduced into this description to describe the position of some elements within the printing machine, we would refer to the terms xe2x80x9coperator sidexe2x80x9d (C.C.) and xe2x80x9cside remote from the operatorxe2x80x9d (C.O.C.) which are used by agreement to refer to a side designated with respect to the central longitudinal axis of the machine. This choice prevents any confusion with the conventional left-hand and right-hand denominations, which depend on the point of view of the observer.